A typical automatic transmission includes a hydraulic control system that, among other functions, is employed to actuate a plurality of torque transmitting devices. These torque transmitting devices may be, for example, friction clutches and brakes. The conventional hydraulic control system typically includes a pump that provides a pressurized fluid, such as oil, to a plurality of valves and solenoids within a valve body. The pump is typically driven by the engine during operation of the powertrain.
However, in the case of hybrid powertrains using a combination of the internal combustion (IC) engine and electric propulsion motor powertrains, the engine has periods of shutdown in order to conserve fuel. As a result, during this time of passive engine operation the main transmission pump stops pressurizing the hydraulic fluid in the transmission or hybrid transmission. However, the components within the transmission must still receive a flow of pressurized hydraulic fluid in order to maintain operability. Current hybrid systems use a motor driven auxiliary pump to deliver a pressurized hydraulic fluid flow to these components, such as the range clutches, in order to keep these components engaged so that the transmission is ready to respond. However, these conventional auxiliary pump systems that are driven by an electric motor may suffer from low system efficiency, may be large in size, and can be expensive.
Accordingly, there is a need in the art for an auxiliary pump system for use in hybrid powertrains that increases efficiency, thereby leading to better fuel economy and allowing for longer engine passive time periods.